Bipolar transistors are electronic devices with two p-n junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, two p-n junctions, i.e. the emitter-base and collector-base junctions, are separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar transistor action.”
If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because of the mobility of minority carriers, i.e. electrons, in the base region of npn transistors, higher frequency operation and higher speed performances can be obtained with npn transistors. Therefore, the present inventors believe npn transistors comprise many of the bipolar transistors used to build integrated circuits. In FIG. 1(a) and FIG. 1(b), prior art npn and pnp bipolar transistors Q0, Q1, in addition to respective base bias circuits, switches P0, N0, collector load circuits and emitter load circuits are shown.
Performance wear-out over a device (bipolar transistor) operating lifetime has been a major problem for all semiconductor devices. One of the wear-out mechanisms is the known hot carrier effect due to reverse bias of the base-emitter junction in bipolar devices. In some circuit applications, the base-emitter junction is required to be reverse biased at a high voltage (such as VBE=1.5 volts or higher) for a long time, which would severely degrade key device performance parameters, in particular a current gain (hfe “aka” β). The current gain β is a ratio of the collector current Ic divided by the base current Ib. The reverse bias of the base-emitter junction at a high voltage for a long time significantly reduces the bipolar transistor operating lifetime.
For discussions of the hot carrier effect and reverse bias of the base-emitter junction in bipolar transistors, see, for example: HOT-ELECTRON-INDUCED DEGRADATION AND POST-STRESS RECOVERY OF BIPOLAR TRANSISTOR GAIN AND NOISE CHARACTERISTICS, by Sun et al., IEEE Transections on Electron Devices, Vol. 39, No. 9, September 1992, pgs. 2178-2180; TEMPERATURE DEPENDENCE AND POST-STRESS RECOVERY OF HOT ELECTRON DEGRADATION EFFECTS IN BIPOLAR TRANSISTORS, by Huang et al., IEEE 1991 Bipolar Circuits and Technology, 5/91, pgs. 170-173.
Traditionally, bipolar device manufacturers and designers are limited by a maximum allowed base-emitter reverse voltage (VBE) in order to ensure reliability in the transistor operation lifetime. However, as the reverse voltage limit continues to be reduced in advanced semiconductor technology with more demanding performance requirements and ever shrinking device dimensions, it is becoming very problematic for circuit designers to use standard circuit libraries developed from previous technologies (e.g., larger technology nodes).
For example, a customer may desire a semiconductor foundry to support a transistor circuit (device) with a reverse bias voltage (VBE) at 3.0V using, for example, BiCMOS6WL technology of IBM that offers a reverse voltage bias limit of approximately (±10%) 1.75V. With traditional reliability guidelines and models, such a design significantly impacts reliability during the transistor lifetime and, thus, is problematic. Meeting reliability requirements for such transistor designs is very challenging. It is, therefore, very important for, for example, a semiconductor foundry or other semiconductor manufacturer to have a solution in order to alleviate this base-emitter junction reverse bias voltage limit.
U.S. Pat. No. 7,238,565 B2 entitled “METHODOLOGY FOR RECOVERY OF HOT CARRIER INDUCED DEGRADATION IN BIPOLAR DEVICES,” by Guarin et al., issued Jul. 3, 2007, discloses the recovery of degradation caused by the hot carrier effect in the base-collector junction using thermal annealing. A method of forward biasing a bipolar transistor for degradation recovery is also disclosed in this patent. In one of several embodiments disclosed in the '565B2 Patent, a high forward current around the peak fT current is provided to the bipolar transistor while operating below an avalanche condition (VCB of less than 1 volt). The high forward current contributes to increase the temperature of the bipolar transistor to about 200° C. or greater. More particularly, appropriately increasing the temperatures of the base-collector junction and the base-emitter junction contributes to recovering the degradation significantly. U.S. Pat. No. 7,238,565 B2 is hereby incorporated in its entirety herein by reference.
The present inventors believe that improvements to the invention disclosed in U.S. Pat. No. 7,238,565 B2, assigned to International Business Machines Corporation (the assignee of the present invention and patent application) are achievable, particularly with respect to implementations (methods and arrangements) for gain recovery in a bipolar transistor.